Medical conditions are often diagnosed or monitored with the assistance of test strips that provide an optical response, (e.g., color) to reactions between analytes in a biological sample (e.g., glucose in whole blood) and reagents disposed on the test strip. Although an optical response can be read visually, frequently they are measured using reflectance photometry. In that method, light is directed onto a reagent-containing area on the test strip and light returned from the test area is detected and correlated with the amount of the analyte reacted on the test strip.
Many patents disclose arrangements of light sources and detectors intended to improve the performance of instruments employing reflectance photometry. Examples include U.S. Pat. Nos. 6,181,417; 5,611,999; 4,890,926; 5,155,628; 5,449,898 and 5,477,326. Typically, LEDs are used to provide a high intensity, narrow band width light source. Light returned from a test strip that has been affected by the optical properties of the test area, referred to as diffuse light, is collected and measured by a photo detector. Returning light, referred to as specular light herein, which is not affected by the test area, is prevented from reaching the light detector.
The light, as it travels between the light source and the light detector, has been handled with various methods familiar to those skilled in the art. Optical fibers have been employed in some methods. An example is found in U.S. Pat. No. 5,701,181 in which an LED supplies light through an optical fiber bundle at an angle of 30 degrees to a pad on a test strip. The reflected light is directed to a light detector via a bi-convex lens positioned behind a threaded light baffle to a second bundle of optical fibers.
Another sensing device of a completely different design is described in U.S. Pat. Nos. 6,157,472 and 6,535,753. It employs optical fibers in a self-contained unit to direct light into a very small sensing tip directly in contact with the subject being tested and to receive light reflected back from the tip. The sensing tip is extremely small and is intended to enter the skin of a patient with little or no sensation being felt. The tip is coated with materials that are selected to react with the analyte to be detected. An example given in U.S. Pat. No. 6,535,753 employs a glucose oxidase reagent system. Optical fibers tipped with reagents have been proposed for measuring other analytes, such as nitric oxide. Another example is found in U.S. Pat. No. 6,636,652.
Typical instruments have a number of limitations that relate to the handling of light. The amount of diffused light returned from the test area is small and therefore the test area must be relatively large, leading to larger equipment than would be desired. The arrangements of the optical elements of the instruments present problems for the designer. Further, eliminating specular light reflections, that is, light that has not been affected by the optical response of the test area, generally is not completely successful. Consequently, further improvements in such instruments are continually sought. The present invention employs fiber optics to overcome limitations found in conventional instruments, as will be seen in the description below.